Summary The ability to categorize objects and events and to extend this categorization behavior to new instances is foundational to many human activities. We sort the objects and events around us into categories, while still being able to recognize some or all of the individual members of each category. The ability to form categories is especially adaptive, because it allows us to respond appropriately to novel stimuli after experiencing only a few instances from a given category. The same ability should also benefit nonhuman animals. Over the years, animal models have dramatically increased our understanding of the brain mechanisms of human learning and memory, attention and attentional disorders, as well as schizophrenia, aggression, and depression. The proposed research will develop effective pigeon and rat models of human categorization at both behavioral and neural levels. Despite appreciable differences from humans, these animal models will enable direct access to behavioral variables and brain structures that would not otherwise be possible. With both pigeons and rats as experimental subjects, we will systematically explore the role of category structure and task supervision on mastery of the same family of categorization tasks that we will also give to human infants, children, and adults as well as computer models. We will train birds and mammals with tasks that are equated for contingencies of reinforcement, category structure, and task structure, rendering this comparison unique to our collaboration. The bird-mammal comparison will further allow us to explore homologous functions of the hippocampus and striatum as well as analogous functions of the prefrontal cortex and the avian nidopallium caudolaterale. Divergences in categorization behavior between birds and mammals along dimensions that are sensitive to damage to these brain structures will indicate how similar these species are and how they may have diverged through evolution. These results should provide key insights into the behavioral and neural mechanism of categorization with particular relevance to normally and abnormally developing children prior to the emergence of language.